1. Field of the Invention
The present invention relates to a switch device, and more specifically a switch device with a testing possibility for testing the operability of an electric switch.
2. Description of the Related Art
In protective circuits, often all components that switch power (switches) are subjected to a function test prior to being activated. By such a function test, errors leading to severe damage or destruction of the components of the circuit during the operation of the circuit can be recognized. In FIG. 2, such a conventional circuit (for example with a switching circuit or a converter) with overcurrent recognition is illustrated. The circuit according to FIG. 2 includes a voltage source 200 with a first terminal 202 and a second terminal 204. Furthermore, the circuit includes a resistor R and a diode D, wherein the diode D comprises a cathode K and an anode A. The resistor R is connected between the anode A and the first terminal 202 of the voltage source 200. Furthermore, the circuit comprises a switch S3 to be tested with a first terminal 206 and a second terminal 208 as well as a control terminal 210. The first terminal 206 of the switch S3 to be tested is conducively connected to the cathode K. The second terminal 208 of the circuit S3 to be tested is conducively connected to the second terminal 204 of the voltage source 200. The control terminal 210 of the switch S3 is conducively connected to a control output 212 of a drive circuit T, wherein the drive circuit T further comprises a first terminal 214 connected to the first terminal 202 of the voltage source 200 and a second terminal 216 connected to the second terminal 204 of the voltage source 200. The circuit according to FIG. 2 further comprises an input with a first input terminal 220 and a second input terminal 222. A capacitor 224 may be connected between the first input terminal 220 and the second input terminal 222. Furthermore, the circuit comprises an output with a first output terminal 230 and a second output terminal 232. Between the first output terminal 230 and the second output terminal 232, as illustrated in FIG. 2, a further diode D1 with a further cathode K1 and a further anode A1 may be connected, wherein the further cathode K1 is conducively connected to the first output terminal 230 and the further anode A1 to the second output terminal 232. Furthermore, the first input terminal 220 is conducively connected to the first terminal 206 of the switch S3 to be tested. The second terminal 208 of the switch S3 to be tested is conducively connected to the first output terminal 230. The second output terminal 232 is conducively connected to a first terminal 240 of a further switch S4, whereas the second input terminal 222 is conducively connected to a second terminal 242 of the further switch S4, wherein the further switch S4 additionally includes a control terminal 244. Furthermore, a switch device 250 is formed by the components of the voltage source 200, the drive circuit T, the resistor R, the diode D, and the switch S3 to be tested.
For a switch device 250 thus connected, there are several possibilities for monitoring the operability of the circuit or for monitoring the operability of the switch S3. For example, if an IGBT (isolated-gate bipolar transistor) is used as switch S3 to be tested, the gate-emitter voltage (voltage between the control terminal 210 of the switch S3 (measuring point G) and the second terminal 204 of the voltage source 200) and the collector sense voltage (voltage between the anode A of the diode D (measuring point C) and the second terminal 204 of the voltage source 200) are available as observable signals. If the gate (i.e. the control terminal 210 of the switch S3 to be tested) is controlled with 0V with reference to the emitter 208, the operable IGBT blocks, and the maximum voltage (i.e. the operative voltage of the drive circuit T or the maximum voltage of the voltage source 200 of 15V, for example) is present at the anode A. If the IGBT is turned on via the drive circuit, (i.e. for example 15V drop at the gate-emitter path) the collector-emitter path conducts, and the voltage that can be tapped at the measuring point C between the anode A and the diode D and the second terminal 204 of the voltage source 200 becomes lower, which can be detected by an on-the-fly testing means, for example. Alternatively, the on-the-fly testing means may also monitor a potential of the first terminal 206 of the switch S3 to be tested. With converters, by monitoring the measuring point C in the turned-on state, a too high collector-emitter current can be recognized. By turning the converter off, thermal destruction of the IGBT can be prevented in this case.
It proves disadvantageous, however, that some errors can indeed be recognized by this diagnosis (e.g. an open in the collector-emitter path), but not all occurring errors. For example, a short between gate and emitter cannot be recognized securely. Only if the short is significantly more low-ohmic than the gate driver, can this error be detected.